With a few exceptions, tradition and technology have favored transillumination pulse oximetry in the operating theater. The principle of operation of the pulse oximeter sensor is fairly simple but is arguably the most important development in anesthesia monitoring in the twentieth century. Two wavelengths of light (usually 660 nm and 940 nm) are used to spectrophotometrically determine the ratio of oxidized to reduced hemoglobin noninvasively as well as to determine the pulsatility of blood plethysmographically. Presently, the most common application of this in the operating theater is via transillumination through the capillary bed of a peripheral digit. However, it is not unusual for multitrauma and thermally injured patients to either have severe peripheral vasoconstriction or to have severely damaged (or missing due to amputation) peripheral vascular beds. Reflectance oximetry rather than transillumination oximetry was the earliest investigative form of the technique. Transillumination pulse oximetry, without question, is the most effective form when oximetry is obtained through skin. However, when skin is not interposed as a barrier to capillary bed access, reflectance pulse oximetry easily can be achieved with very accurate results. The effect is achieved by the backscattering of incident bispectral light that traverses and, on reflection from nonabsorptive collagenous tissues, retraverses formed elements in the blood back to the oximetric detector. Rather than superseding transillumination pulse oximetry, this technique broadens the scope of possible monitoring sites, adding to the clinician's armamentarium.
Conventional pulse oximetry in the severely burned patient can be a significant challenge, yet these data are vital in operating room and intensive care settings. Most current oximetric approaches depend upon available peripheral sites permitting transillumination oximetry and indeed, this method is sufficient for most surgical conditions and procedures. Unfortunately, patients with severe burns often have few sites for the effective placement of the transilluminating pulse oximeter sensor. In addition, these patients often have severe circulatory compromise rendering the peripheral pulse oximeter less efficient. A variety of studies have shown that oral pulse oximeter sensors are more reliably and rapidly responsive than peripheral pulse oximeter sensors. However, many of these studies use oral transillumination pulse oximetry, held in place via complex devices or pieces of improvised malleable metal. Oral secretions, equipment failure, and placement difficulty often render these techniques ineffective.
Reflectance oximetry can be a useful tool where a capillary bed is easily accessible. Indeed, it is used commonly and effectively among intrapartum and neonatal patients whose capillary beds are easily accessed through their skin. The technique has also been applied to adult and pediatric burn patients by placing the reflectance sensor in wounds or over hyperemic sites such as healed partial thickness burns. There remain a variety of other useful capillary beds that will provide better and more accurate data and that are not easily attainable with current disposable pulse oximeter sensors.
Bodily fluids that come in contact with conventional, off-the-shelf disposable pulse oximeter sensors upon insertion into an open wound or body cavity negatively effect the operation of the pulse oximeter sensors. Disposable pulse oximeter sensors, like the Nellcor.RTM. Oxisensor.RTM. II D-25, will begin to electrically malfunction over time, because liquid from the body cavity or wound will begin to breakdown the protective material around the electrical components and then seep into the area around the electrical components and short circuit the disposable pulse oximeter sensor causing the sensor to malfunction.
It is difficult to predict when a disposable pulse oximeter sensor will malfunction due to exposure of its electrical components. Accordingly, a need exists for protecting off-the-shelf disposable pulse oximeter sensors with a disposable protective covering for use in areas with fluids.